1. Field of the Invention
This invention relates to a two-laser beam optical head used for radiating light power detection optical systems and information signal detection optical systems of optical information processing apparatus such as a disk apparatus, optical card apparatus or optical tape apparatus.
2. Description of the Prior Art
In an optical disk apparatus or the like, an error ratio at the time of writing of information is generally higher than in a magnetic disk apparatus. Accordingly, in a one-beam optical head consisting of one light source, it has been customary to check the writing error at the next rotation of the disk after writing and to write once again for the writing error portion. For this reason, it has been necessary to rotate the disk twice before writing is complete. Therefore, Japanese Patent Laid-Open No. 150147/1986, for example, proposes a two-beam laser optical head consisting of two light sources so that the error check can be made immediately after writing. According to this prior art reference, two semiconductor laser beams having different wavelengths are converted to parallel beams by use of two collimate lenses, respectively, and are then synthesized by a beam splitter, and two spots are focused on one track by use of a focusing lens so that the information is written by the preceding laser spot while the information is read by the other laser spot immediately after writing. The reflected beam of the writing laser beam from the disk is guided to a conventional focus error detection optical system and a conventional track error detection optical system through a filter which transmits only the wavelength of the laser described above so as to detect spot position signals such as a focus error signal, a track error detection signal, and an index signal, etc. The reflected beam of the reading laser beam from the disk is guided to a conventional read signal detection optical system through another filter which transmits only the wavelength of the reading laser beam in order to read the information.
Furthermore, since the radiating light power of the semiconductor laser changes with temperature and time, the radiating light power must be detected always and fed back to a driving current. The laser beam is emitted in two directions, that is, forward and rearwards, from a semiconductor laser chip. Therefore, in a package to which only one semiconductor laser chip is mounted, a monitor detector is fitted in advance at the rear part of the chip. Various chips of this type are available commercially on the market. Though it is not mentioned in the prior art reference described above, two semiconductor lasers having different wavelengths have independent packages. Therefore, the semiconductor laser equipped with the monitor detector described above can be employed.
In Japanese Patent Laid-Open No. 929/1986, on the other hand, the writing laser and the reading laser of the two-laser beam optical head are independent and an optical band pass filter is rotated for adjustment so that the writing spot is ahead of the reading spot with a predetermined gap between them. In order to locate both the writing spot and the reading spot on the same track on the disk, a method has been employed which rotates and adjusts a module as an integral unit of the writing laser, a collimate lens and a beam shaping prism.
The first prior art technique described above requires two semiconductor packages each having a monitor detector, two collimate lenses, three beam splitters, two optical band pass filters and at least two optical detectors. Since the number of optical components is great, the optical head is heavy and its cost is high. The second prior art technique requires the adjustment of the writing laser and the reading laser fitted separately from each other and the adjustment of their spot gap, and a large number of optical components necessary as in the first prior art technique. Though this problem can be solved by use of an array laser having two lasers stored in one package, another problem occurs in that since the two laser sources are stored in one package, locating the same track on the disk plane cannot be done by moving only one of the laser sources as in the prior art technique.
To solve this problem, a possible solution would be the rotation of the two-laser beam optical head as a whole so as to locate the two spots on the same track. Here, an example shown in FIG. 14 of the accompanying drawings will be explained. As shown in (a), if two spots 38, 39 are not on the same track on an inner track 34, the two spots 38 and 39 can be put on the track 34 by inclining two optical head feed shafts 32 and rotating the optical head 31 as shown in (b), but if the optical head 31 is moved as such on an outer track 35 as shown in (c), both of the two spots 38, 39 are not on the outer track 35 due to the difference of radius of curvature of the inner and outer tracks of the disk. This is because a deviation 37 exists between a perpendicular line 36 passing the center of a line 43 connecting the two spots 38, 39 and the center 33 of the circle of the tracks.